The present invention relates to planar conductors for use in printed circuit boards, multichip modules, and integrated circuits, and micro electromechanical devicesxe2x80x94and methods for making such inductors.
Devices having inductive cores are important in electric and electronic apparatuses. Such devices include transformers, inductors, and electromagnets. In the past it is been relatively difficult and expensive to produce inductive devices of the scale appropriate for use in many printed circuit boards, multichip modules, integrated circuits, or micro electromechanical (xe2x80x9cMEMxe2x80x9d) devices. In the past such small inductive devices usually have been fabricated using a variety of mechanical operations. These operations have included stamping, or micromachining ferromagnetic materials suitable for use in as cores and then winding primary and/or secondary coils around them. Usually such prior art inductive devices have been separate units that have to be individually mounted upon printed circuit boards or multichip modules. Such devices are often much larger in volume than many other electronic devices and, thus, it has long been a desire to reduce the size of such inductive devices.
In the prior art there has been some creation of ferromagnetic cores upon integrated circuits for use in the micro electromechanical devices. In the past most such ferromagnetic cores have been deposited on the MEM devices by sputtering, which is a relatively expensive, time-consuming, and high-temperature process. The high-temperature of sputtering processes makes them impossible to use on many types of substrates or dielectrics.
It is an object of the present invention to provide inductive cores for use in inductors, transformers, and electromagnets which can be manufactured inexpensively, which consume relatively little space, and/or which can be formed on many different substrates.
According to a first aspect of the present invention a printed circuit board is provided comprised of two layers of printed circuit board dielectric material; a core made of ferromagnetic material between the two layers; and conductive leads on the opposite side of each dielectric layer from the core which are connected by via holes through both dielectric layers so as to form a conducting coil around the core.
In some embodiments of this aspect of the invention the conductive leads form two separate coils around the same core so that one such coil can act as a primary winding and the other can act as a secondary winding of a transformer.
In some embodiments a planar conducing sheet is placed on or between one or more of the printed circuit board""s dielectric layers so as to shield other circuitry on the printed circuit board from magnetic fields generated around the core.
In some embodiments the core is formed of two separate layers of ferromagnetic material, each formed at a corresponding location on a facing side of one of said two dielectric layers.
In some embodiments the core has been formed at least in part by electroless plating on one or both of said dielectric layers. In some such embodiments the core has been deposited on one or more of dielectric layers by the following process: dipping the surface of the dielectric material in a solution containing catalytic metal particles which have a slight electrostatic dipole when in solution to help those particles attach to the dielectric material""s surface; and placing the surface of the dielectric material in a first metal salt solution in metastable equilibrium with a reducing agent so as to cause a first layer containing metal to be plated upon the surface of the dielectric material containing the catalytic metal particles by a process of electroless plating. The electroless plating process can further include, before dipping the dielectric material in the solution of catalytic metal particles, plasma etching the surface of the dielectric material to roughen its surface and to create peaks and valleys in the surface of that material which have van der Waal forces capable of attracting catalytic particles which have a slight electrostatic dipole. Such etching is not necessary if the surface of the material upon which the cores the deposited already has a texture which generates the appropriate van der Waal forces. For example FR4, in material commonly used makes layers are printed circuit boards usually has a texture which is not require such etching to prepare it for the electroless plating process.
In some embodiments of this aspect of the invention the first layer put down at the start of the electroless plating process is thin layer of relatively conductive metallic material used as a seed layer. This is often done because more metallic, and thus more conducting, materials often, are more easily prompted out of the metastable solution by the catalytic particles.
In such embodiments once such a thin seed layer has been established the surface of the dielectric material can then be placed in a second metal salt solution in metastable equilibrium with a reducing agent which has a higher concentration of non-metal elements than the first solution so as to cause material to be plated upon the seed layer which is a ferromagnetic material having a lower conductivity than the material of the seed layer.
In some embodiments such a relatively conductive seed layer is used as an electrode so that electroplating can be used to plate a thicker layer of less conductive ferromagnetic material onto the seed layer.
According to another aspect of the present invention a method is provided of forming ferromagnetic inductive cores on the surface of a dielectric material. This method comprises the following: dipping the surface of the dielectric material in a solution containing catalytic metal particles which have a slight electrostatic dipole when in solution to help those particles attach to the dielectric material""s surface; and placing the surface of the dielectric material in a first metal salt solution in metastable equilibrium with a reducing agent so as to cause a first layer containing metal to be plated upon the surface of the dielectric material containing the catalytic metal particles by a process of electroless plating.
Some embodiments of this method further include, before dipping the dielectric material in the solution of catalytic metal particles, plasma etching the surface of the dielectric material to roughen its surface and to create peaks and valleys in the surface of that material which have van der Waal forces capable of attracting catalytic particles which have a slight electrostatic dipole. As stated above such plasma etching is not always necessary because some materials upon which such cores are deposited already have the proper surface roughness. In other embodiments of the indention where the material needs roughening other methods of roughening the surface can be used including chemical etch and/or mechanical abrasion. When a plasma etch is used for this purpose it is commonly a non-reactive ion etch.
In some embodiments of this method the ferromagnetic material deposited is phosphorous doped nickel. In others the ferromagnetic material is boron doped nickel.
In some embodiments of this method the catalytic particles are particles of one of the following metals: cobalt, palladium, ruthenium, rhodium, platinum, iridium, osmium, nickel, or iron.
According to another aspect of the present invention a method of forming ferromagnetic inductive cores on the surface of a dielectric material is provided. This method comprises the following steps: dipping the surface of the dielectric material in a solution containing catalytic metal particles which have a slight electrostatic dipole when in solution to help those particles attach to the dielectic material""s surface; and placing the surface of the dielectric material in a first metal salt solution in metastable equilibrium with a reducing agent so as to cause a first layer containing metal to be plated upon the surface of the dielectric material containing the catalytic metal particles by a process of electroless plating.
Some embodiments of this method further include the plasma etching process described above.
Some embodiments of this method change the metal salt solution used in the electroless plating process after a first relatively conductive thin seed layer has been deposited to one which will cause the subsequently deposited material to have a lower conductivity than the seed layer, as has been discussed above.
Some embodiments of this method used such a thin relatively conductive seed layer as an electrode upon which to plate less conductive from magnetic material, as has also been discussed above.
This method can be used to form an inductor core on or between one or more dielectric layers of a printed circuit board, of a multichip module, of an integrated circuit, or of a micro-electromechanical device in which the inductor is used to provide electromagnectically induced movement to a mechanical element of that device.